In order to establish a data connection to a terminal device in a cellular mobile network, the terminal device first has to be located, that means, it first has to be ascertained in which radio cell of the mobile telecommunications network it is located. For this purpose, before a requested data connection is established, a terminal device tracking call is sent out by the network and the terminal device then responds to this tracking call by indicating the radio cell in which it is currently located. The transmission of the terminal device tracking call is also referred to as paging.
Since a very large bandwidth of the paging channel would be necessary if the terminal device tracking call were to be transmitted throughout the entire network, adjacent radio cells are combined to form tracking areas, and, when a terminal device enters a tracking area, it transmits a locating message to the network indicating this tracking area. Moreover, locating messages are normally transmitted by a terminal device to the network at regular time intervals. Thus, the network has knowledge about the tracking area in which the terminal device is located, and the paging can be restricted to this tracking area. The smaller the tracking areas are, that is to say, the fewer radio cells they comprise, the smaller the bandwidth requirement of the paging channel, since, usually, fewer terminal devices are registered in smaller tracking areas and consequently fewer terminal device tracking calls have to be transmitted simultaneously. At the same time, however, in smaller tracking areas, a larger number of locating messages are transmitted since terminal devices often change from one tracking area to another tracking area. Thus, the size of the tracking areas results from a compromise between the bandwidth requirement of the paging channel and the frequency of locating messages.
The above-mentioned tracking areas are referred to in GSM mobile telecommunications networks (GSM: Global System for Mobile Communications) as location areas (LAs) and, within the scope of the GPRS standard (GPRS: General Packet Radio Service), as routing areas (RAs), a location area generally comprising one or more routing areas. Within the radio network referred to as UTRAN (UMTS Terrestrial Radio Access Network), according to the UMTS standard, location areas are provided as tracking areas in the circuit switched part and routing areas are provided as tracking areas in the packet switched part, as long as the terminal device is in the so-called idle state.
Currently, mobile telecommunications systems of the next generation are being developed or standardized under the designation LTE/SAE (LTE: Long Term Evolution, SAE: System Architecture Evolution), and they comprise a radio network with the designation E-UTRAN (Evolved UTRAN). Regarding E-UTRAN, the above-mentioned tracking areas according to Chapter 7.3 of the technical report TR 23.882 “3GPP System Architecture Evolution Report on Technical Options and Conclusions” of the 3GPP are referred to as tracking areas (TAs), and a tracking area can particularly be a location area or a routing area.
Also in view of a change of a terminal device from the E-UTRAN into another network such as, for example, into the UTRAN or vice versa, it has also been proposed to simultaneously associate a terminal device with a tracking area of the E-UTRAN and with a tracking area of the other network such as, for example, a location area or a routing area of the UTRAN. In this case, the transmission of a locating message from the network is only provided if the terminal device enters a tracking area of the E-UTRAN or of the other network that had previously not been associated with it, whereas when a change is made from a tracking area that is associated with the terminal device into another tracking area that is associated with it, no signal is sent to the network. In this manner, it can be avoided that a signalling has to occur in case of a change between the networks. Furthermore, it was proposed that several tracking areas of the E-UTRAN are associated with a terminal device at the same time, and likewise, a locating message is only sent to the network if the terminal device enters a tracking area that had not been associated with it previously.
The tracking areas simultaneously associated with the terminal device are referred to as equivalent tracking areas (ETAs). The concept of the equivalent tracking areas is described in Appendix D.2.4.1 of the technical report TR 23.882 of the 3GPP and in the 3GPP document R3-060457 (see http://www.3gpp.org/ftp/tsg_ran/WG3_Iu/-TSGR3—51bis/docs/R3-060457.zip).
In the concept of the equivalent tracking areas, the network does not know in which of the equivalent tracking areas the terminal device is located. According to the technical report TR 23.882, Appendix D.2.4.1, the paging then takes place correspondingly in all equivalent tracking areas. However, this involves a very high network load and leads to a very large bandwidth requirement of the paging channel.